Field of the Invention
The invention concerns techniques for slice-selective magnetic resonance imaging of a target slice of an examination subject, as well as a magnetic resonance system. In particular, the invention concerns techniques of the type that enable a reduced measurement time and/or radio-frequency radiation exposure, for example based on a Slice Encoding for Metal Artifact Correction (SEMAC) sequence.
Description of the Prior Art
In magnetic resonance (MR) imaging, nuclear spins in a subject are aligned or polarized by the application of a basic magnetic field, and that nuclear spins are subsequently by radiation of one or more radio-frequency (RF) pulses, deflected out of the steady state, or are specifically manipulated (thus refocused, for example). The locally polarizing magnetic field may exhibit inhomogeneities, i.e. fluctuations as a function of location. For example, this can be due to structurally dependent inhomogeneities of the basic magnetic field and/or due to the presence of susceptibility changes as a function of location. For example, such susceptibility changes can occur due to metal articles in the examination region, such as prostheses or surgical elements.
These inhomogeneities can produce image artifacts in MR images, for example, because the local resonance frequency of the nuclear spins is displaced due to the inhomogeneities and incorrect mappings thereby occur in the MR data. A defined point in space thus might be mapped to a different point in an MR image.
In order to suppress metal artifacts in spin echo (SE)-based measurement sequences, a Slice Encoding for Metal Artifact Correction (SEMAC) technique can be used; see “SEMAC: Slice Encoding for Metal Artifact Correction in MRI”, W. Lu et al. Magn. Reson. in Med. 62 (2009) 66-76. In a SEMAC sequence an additional phase coding in the slice selection direction kz is typically implemented in connection with a conventional two-dimensional (2D) measurement sequence, or slice selective scanning of an examination subject.
Two effects can occur in connection with such SEMAC techniques. First, the entire time period (measurement time) required to acquire MR data typically increases linearly with the number of additional phase coding steps in the slice selection direction kz. This can limit the flexibility in the imaging and may produce movement artifacts or the like. Economy of operation of the MR system can simultaneously be limited. Second, an RF load produced by the MR imaging can increase, which is often quantified by a designation known as the specific absorption rate (SAR). This is typically the case because a large number of refocusing pulses with RF portion are radiated per time unit. Therefore, it may be necessary to provide additional downtimes in order to limit the SAR, thereby additionally lengthening the measurement time.